The increased use of telephone twisted pair wiring for data communications has resulted in a push for faster modems and improved signaling protocols compatible with the public switched telephone network (PSTN). Examples of such improved protocols include the emerging modem communication standards which have pushed the limit of transmission speeds close to 56 Kbps. Other examples include the emerging variety of digital subscriber line (DSL) communications protocols--including asymmetric digital subscriber line (ADSL), symmetric digital subscriber line (SDSL), hi-bit rate digital subscriber line (HDSL), and very high rate digital subscriber line (VDSL). Each DSL variant represents a different transmission speed over a different distance of copper pair wiring. Currently, industry is manufacturing communication equipment capable of implementing one or more of the DSL protocols. Modems, routers, line cards, and digital loop carrier systems are all examples of such equipment.
Theoretically, a DSL modem and a plain old telephone system (POTS) or other voice band device should be able to operate simultaneously over the same wire line pair since they use different frequency bands. Splitterless modems try to take advantage of this principle to transmit both voice band and digital DSL signals simultaneously. An example of such a splitterless modem architecture is disclosed in related U.S. patent application Ser. No. 09/216,082 (the "Related Application"), entitled "Residential Power Cut-back for Splitterless DSL Operation".
The Related Application discloses a splitterless DSL modem capable of operating in at least two steady states: one during which normal DSL data transmission occurs and another in which voice transmissions occur simultaneously with DSL. The modem can also operate in a "transient" state that occurs in the brief time it takes to transition from one of the first steady states to another.
Unfortunately, the connection of a POTS to the same wire line pair as a DSL modem can suffer from several problems that occur during the presence of transient signals. One transient, called the "off-hook transient" can cause data disruptions and/or a disconnect from the line. Typically, the off-hook transient occurs as the POTS device coupled to the same wire line pair goes from the on-hook condition (handset down) to the off-hook condition (handset up) or state. Thus, the activation of the POTS device creates an off-hook transient that may appear as a spike or other signal form over the wire line pair coupling both the DSL modem and the POTS device to the central office. Other transients, including on-hook and ring signals may cause similar problems.
The Related Application discusses other changes in line conditions such as a change in the line impedance as the POTS changes states. While such changes in line condition can be short-lived, they oftentimes interrupt or disconnect the DSL modem from the line and cause data to be lost. It would therefore be advantageous to have a system and method of accommodating changes in line condition that occur as a POTS device coupled to the same line as the DSL modem goes from one steady state to another. A system and/or method of preventing data loss and/or disconnects due to transients would provide many benefits in the context of a DSL environment.